A polymer having a wide molecular weight distribution is generally applicable to the production of industrial pipes, films and sheets, whereas a polymer having a narrow molecular weight distribution is used for injection molding. The above two types of polymers are clearly distinguished by the difference in molecular weight distribution, determined by the ratio of weight average molecular weight to number average molecular weight, and by the difference in melt flow rate.
A polymer polymerized in a slurry or vapor single reactor in the presence of a Ziegler catalyst generally has a narrow molecular weight distribution. Such polymers having a narrow molecular weight distribution have limited tensile strength, which causes deformity during processing. In addition, they are not appropriate for a processing method requiring high mechanical resistance in melting point.
Approaches have been made to prepare a polymer having a wide molecular weight distribution using a Ziegler catalyst. As an example, Zucchini U. and G. Cecchin reported a method for subsequent or gradual polymerization in the presence of a Ziegler-Natta catalyst using at least two different reactors to prepare a polymer having a wide molecular weight distribution (Adv. In Polymer Science 51, 101˜153 (1983)). However, the process of this method is complicated and thus is likely to cause problems in the production of a real product. European Patent No. 658577 (Himont) describes a high-stereoregular polypropylene having a wide molecular weight distribution prepared using two reactors.
Regarding a catalyst for the preparation of a polymer having a wide molecular weight distribution, Altemore et al described in U.S. Pat. No. 3,8909,477 the use of titanium halide, vanadium halide and an organic aluminum compound as a mixed catalyst. Particularly, according to this description, the co-treatment of allyl aluminum sesqui ethoxide and trialkyl aluminum with a catalyst before polymerization can generate a polymer having a wide molecular weight distribution. However, this catalyst preparation is very complicated and it is also very difficult to control the polymerization since the reactivity of the titanium and vanadium sources, the monomers and the comonomers are different.
To improve fluidity, olefin polymers having different molecular weight distributions have been polymerized in different reactors and then mixed. However, this method requires a long production time and produces uneven products. According to the recent report by Mitsui Petrochemical Corp. (Japan) (Korean Patent Publication No. 1990-0014436), an olefin polymer having a wide molecular weight distribution can be produced by using at least two specific electron donors, in which the ratio of melt flow rate (MFR) of homo-polymers prepared under the same polymerization conditions is higher than 31.6. However, the activity of a catalyst used in this method is very low, suggesting that regulation of the molecular weight distribution of the polymer is difficult, which means the product might not be commercially viable and the processing itself is limited by the low hydrogen reactivity involved in the regulation of the polymer's melt flow rate (MFR). U.S. Pat. No. 5,652,303 by Mitsui Petrochemical Corp. also describes that molecular weight distribution and tacticity can be regulated by using at least two external electron donors.
Pre-polymerization methods for the preparation of polymers or copolymers having excellent hydrogen reactivity and tacticity, which use a titanium catalyst containing at least three different carbon atoms, specifically electron donor treated magnesium and solid complex titanium containing titanium and a halogen, for the polymerization or copolymerization of alpha-olefin have been proposed (Japanese Patent Publication No. 73-16986, No. 73-16987 and German Patent Publication No. 2,153,520, No. 2,230,672, No. 2,230,728, No. 2,230,752 and No. 2,553,104).
These pre-polymerization methods include the processes of mixing the catalyst components and forming a catalyst. The characteristics of the solid titanium containing catalyst depend on the mixing and forming conditions. Therefore, it is impossible to expect similar results from different conditions. Sometimes a catalyst with poor quality might be produced. Even if a catalyst is prepared under the required conditions, the activity of the catalyst or tacticity of a polymer will not be satisfactory without the addition of external electron donors.
The solid complex titanium catalyst containing minimum levels of magnesium, titanium and a halogen is also affected by the added electron donor. When an alpha-olefin containing at least three carbon atoms is polymerized or copolymerized in the presence of hydrogen and a catalyst composed of titanium and an organic metal compound (family 1-family 4 metals of the periodic table), the results might be unexpectedly changed by the addition of the electron donor. Particularly, when a catalyst composed of titanium trichloride which is converted from titanium tetrachloride using a metal aluminum, hydrogen or an organic aluminum compound is used together with the electron donor, the results of polymerization vary with the kind of electron donor that is added. Thus, it seems that the electron donor is not a simple additive but an important factor involved in the construction of the microstructure of a solid complex catalyst by binding magnesium and titanium compounds sterically and electronically.
In general, pre-polymerization indicates the process of forming a thin olefin film on a catalyst at a moderate reaction temperature and low monomer concentration. At this time, conventional solid titanium catalysts for olefin polymerization are acceptable as the pre-polymerization catalyst, and a Ziegler-Natta catalyst is an example, as described in U.S. Pat. No. 4,482,687, No. 3,642,746, No. 3,642,772, No. 4,158,642, No. 4,148,756, No. 4,447,639, No. 4,518,706, No. 4,866,022, No. 5,103,702, No. 5,124,297, No. 4,330,649, European Patent No. 131,832, Japanese Patent Publication No. 63-54004.
The effects of the pre-polymerization are as follows. First is the rate enhancement effect, which means active species are increased by the pre-polymerization and thus the activity increases. During the pre-polymerization, new active sites are formed in a catalyst or a proper ligand is formed by alpha-olefin, leading to the activation of dormant sites. Second is the molecular specification effect. In the pre-polymerization, aspecific sites are capsulated by a polymer in the early stage of polymerization and thus deactivated, leading to the increase of the early stage reaction speed for isotactic fraction and the improvement of tacticity. Third is the morphology improvement effect. The pre-polymerization results in the even fractionation of a catalyst, suggesting that regular sized and shaped particles are formed, without coagulation of the catalyst. As a result, fines are reduced, whereas bulk density is increased, and therefore particle size distribution is regular and morphology is improved. As explained hereinbefore, such pre-polymerization helps polymerization to improve the desired properties of a polymer with a simple process.
However, the entire mechanism of the pre-polymerization has not yet been explained. It was difficult to regulate pre-polymerization conditions with a Ziegler catalyst to generate a polymer having improved molecular weight distribution, hydrogen reactivity and tacticity.